During both prenatal and postnatal life, there are many factors which shape the developing nervous system. Among these factors are the hormones which are produced both by the mother and by the fetus or neonate itself. Several of these hormones, including thyroid hormone, have been shown to play critical roles in regulating neural development, and subsequent behavior and learning ability. The overall aim of the study is to examine the critical regulatory role of thyroid hormone on the development of the nervous system. This hormone has been shown to regulate aspects of neuronal development in various regions of the nervous system. Deficiencies of thyroid hormone during critical periods of development produce a profound impairment of mental functioning (Cretinism). In order to understand better the specific aspects of neuronal development which are affected by abnormal thyroid states, we will employ a developing neuronal system in which, unlike in more complex regions of the central nervous system, precise analyses can be made of the number of neurons produced during the period of proliferation, the number of neurons lost during the subsequent period of naturally occurring neuron death, the number of synapses which are formed, and the maturation of the dendritic tree. The neuronal system to be employed is the superior cervical sympathetic ganglion (SCG) of the rat. This sytem may be employed as a sensitive morphological assay for the effects of altered hormonal levels during two critical times during development. The first critical time is the period of neuronal proliferation, which in the rat SCG occurs between embryonic day 15 and birth. The second is the period of rapid synaptogenesis, which occurs between birth and the third postnatal week. This period also coincides with the period of normally occurring neuronal death in the SCG.